Many thin image display devices as typified by displays, i.e., flat panel displays, have been released on the market, because they are thinner than cathode-ray tube displays and they do not occupy much space in depth. Their market price has decreased year by year with advances in production techniques, resulting in a further increase in demand and a yearly increase in production. Especially, color LCD TVs have almost become the mainstream of TVs. Also in recent years, organic light-emitting display devices such as organic EL displays, which emit light by themselves and thereby have high visibility, have received attention as the next generation of image display devices. In relation to the performance of these image display devices, there is a strong demand for a further increase in image quality, such as an increase in contrast and color reproducibility, and a decrease in power consumption.
A color filter is used in these liquid crystal display devices and organic light-emitting display devices. For example, in the case of color LCDs, the amount of light is controlled by using a back light as the light source and electrically driving the liquid crystal. Colors are represented by the light that passed through the color filter. Accordingly, the color filter is indispensible for color representation in LCD TVs and plays a large role in determining display performance. In organic light-emitting display devices, a color image is formed in the same manner as liquid crystal display devices, when the color filter is used in combination with an organic, white light-emitting element.
A recent trend is that there is a demand for power-saving image display devices. To increase backlight use efficiency, there is a very high demand for high-luminance color filters. This is a major issue especially for mobile displays such as mobile phones, smart phones and tablet PCs.
Even though technological advances have increased battery capacity, there is still a limit on battery capacity of mobile devices. Meanwhile, there is a trend that power consumption has grown with the increase in screen size. Image display devices containing a color filter determine the design and performance of mobile terminal devices, because they are directly linked to the usable time and charging frequency of mobile terminal devices.
In general, a color filter has a transparent substrate, color layers made of color patterns of the three primary colors (red, green and blue), and a light shielding part formed on the transparent substrate so as to define each color pattern.
To form such color layers, a pigment dispersion method in which pigments with excellent heat resistance and light resistance are used as colorants, has been widely used. However, it is difficult for color filters produced by use of pigments to satisfy the latest demand for higher luminance.
As a means to achieve higher luminance, photosensitive resin compositions for color filters, which contain dyes, have been studied. Compared to pigments, dyes generally have a higher transmittance and are able to produce a high-luminance color filter. However, dyes are problematic in that they are inferior in heat resistance and light resistance to pigments and the chromaticity is likely to change when they are heated at high temperature in color filter production process, for example. Also, photosensitive resin compositions containing dyes have such a problem that aggregates are likely to be precipitated during drying process. Aggregates precipitated in a coating film lead to a remarkable deterioration in contrast and make it difficult to use the coating film as a color layer.
In Patent Literature 1, a specific color photosensitive resin composition containing a colorant, which contains a specific dye and a pigment, and a specific solvent is disclosed. According to Patent Literature 1, it is described that a coating film with high heat resistance and less coating unevenness is obtained from the color photosensitive resin composition.
In Patent Literature 2, a specific blue photosensitive resin composition containing, as a colorant, an organic solvent-soluble dye having a specific structure and an organic pigment is disclosed. According to Patent Literature 2, it is described that a color filter with high lightness is obtained by using the color photosensitive resin composition.
The specific dyes of Patent Literatures 1 and 2 are dissolved in solvents and then used. They are insufficient in heat resistance and light resistance. They are also insufficient in stability, since they contain polar solvents for dissolving the dyes.
Disclosed in Patent Literatures 3 and 4 include: specific color materials containing divalent or higher anions and divalent or higher cations, in which dye skeletons are crosslinked by crosslinking groups, and color filters using the color materials. It is disclosed that the color materials are excellent in heat resistance, and the color filters using the color materials have high contrast and are excellent in solvent resistance and electric reliability.
In Patent Literature 5, a dye dispersion liquid in which a specific dye is dispersed in a solvent with the aid of a specific dispersant, and so on are disclosed. It is described that by using the dye dispersion liquid, no aggregates are precipitated during coating film formation, and a color filter having high luminance, as well as excellent heat resistance and light resistance, can be obtained.